Hydrogeological survey assessing arsenic and other groundwater contaminants in the lowlands of Sumatra, Indonesia

Groundwater conditions in the lowlands of Sumatra, where peat swamps are the dominant landscape, were investigated. Based on topography, soil and geological surface properties, this large area (about 100,000 km2) is vulnerable to groundwater As enrichment under reducing aquifer conditions. The reconnaissance groundwater survey was conducted in the province of South Sumatra, covering both presumed low- and high-risk areas of As enrichment. Five distinct types of groundwater were recognized, reflecting a variety of geological sources and chemical conditions which are understood to be typical for the whole east coast of Sumatra. Groundwater collected from tubewells in the youngest (Holocene) swamp deposits had elevated As concentrations (>10 μg L−1) with a maximum of 65 μg L−1. Other elements exceeding the WHO drinking water guideline values include B, Mn, and Se. In contrast to large deltas of continental South and SE Asia, significantly lower sediment loads are transported by the rivers of Sumatra. The organic-rich Holocene sediments are hence relatively thin. Tubewells tapping the oldest geological formations of the study area (middle Miocene to Pliocene) have a broad range of redox conditions reflecting variations in aquifer geochemistry. This group is generally characterized by alkaline pH conditions and high contents of Na, B, Se, and Sr. Oxic groundwaters were found in regions elevated above 20 m a.s.l. and are characterized by low concentrations of dissolved solids and acidic pH values (average 5.1). To date, groundwater data for the increasingly populated island of Sumatra are non-existent in the international literature and this study thus provides a basis for future in-depth groundwater studies. The complete georeferenced database of groundwater analysis is provided as supplementary material.